Add ASCII renderer optimizations

Author: Shane32

QRCoder/ASCIIQRCode.cs

@@ -46,7 +46,6 @@ public string GetGraphic(int repeatPerModule, string darkColorString = "██",
     /// <returns></returns>
     public string[] GetLineByLineGraphic(int repeatPerModule, string darkColorString = "██", string whiteSpaceString = "  ", bool drawQuietZones = true)
     {
-        var qrCode = new List<string>();
         //We need to adjust the repeatPerModule based on number of characters in darkColorString
         //(we assume whiteSpaceString has the same number of characters)
         //to keep the QR code as square as possible.
@@ -55,9 +54,34 @@ public string[] GetLineByLineGraphic(int repeatPerModule, string darkColorString
         var adjustmentValueForNumberOfCharacters = darkColorString.Length / 2 != 1 ? darkColorString.Length / 2 : 0;
         var verticalNumberOfRepeats = repeatPerModule + adjustmentValueForNumberOfCharacters;
         var sideLength = (QrCodeData.ModuleMatrix.Count - quietZonesModifier) * verticalNumberOfRepeats;
+        var qrCode = new string[sideLength];
+        var lineCapacity = (QrCodeData.ModuleMatrix.Count - quietZonesModifier) * repeatPerModule * darkColorString.Length;
+#if HAS_SPAN
+        if (darkColorString.Length == whiteSpaceString.Length && lineCapacity < 510)
+        {
+            Span<char> row = stackalloc char[lineCapacity];
+            for (var y = 0; y < sideLength; y++)
+            {
+                var offset = 0;
+                for (var x = 0; x < QrCodeData.ModuleMatrix.Count - quietZonesModifier; x++)
+                {
+                    var module = QrCodeData.ModuleMatrix[((y + verticalNumberOfRepeats) / verticalNumberOfRepeats - 1) + quietZonesOffset][x + quietZonesOffset];
+                    for (var i = 0; i < repeatPerModule; i++)
+                    {
+                        var sourceSpan = module ? darkColorString.AsSpan() : whiteSpaceString.AsSpan();
+                        var targetSpan = row.Slice(offset, darkColorString.Length);
+                        sourceSpan.CopyTo(targetSpan);
+                        offset += darkColorString.Length;
+                    }
+                }
+                qrCode[y] = row.ToString();
+            }
+            return qrCode;
+        }
+#endif
+        var lineBuilder = new StringBuilder(lineCapacity);
         for (var y = 0; y < sideLength; y++)
         {
-            var lineBuilder = new StringBuilder();
             for (var x = 0; x < QrCodeData.ModuleMatrix.Count - quietZonesModifier; x++)
             {
                 var module = QrCodeData.ModuleMatrix[((y + verticalNumberOfRepeats) / verticalNumberOfRepeats - 1) + quietZonesOffset][x + quietZonesOffset];
@@ -66,9 +90,10 @@ public string[] GetLineByLineGraphic(int repeatPerModule, string darkColorString
                     lineBuilder.Append(module ? darkColorString : whiteSpaceString);
                 }
             }
-            qrCode.Add(lineBuilder.ToString());
+            qrCode[y] = lineBuilder.ToString();
+            lineBuilder.Length = 0;
         }
-        return qrCode.ToArray();
+        return qrCode;
     }
 
     /// <summary>